This invention relates to frequency testing apparatus for determining the natural frequencies of brush seal segments under pressure and/or simulated rotor interference and methods therefor.
Brush seals are increasingly utilized in more challenging conditions such as large steam and gas turbines and aircraft engines to seal between stationary and rotary components. Brush seals typically comprise a plurality of bristles formed of ceramic or metal materials extending from one or more backing plates, terminating in free ends. A brush seal conventionally extends between components to be sealed, e.g., extending from a stationary component typically with the free ends of the bristles engaging the rotary component. Also, as conventional, the proximal ends of the bristles are disposed between a pair of backing plates and welded to the backing plates. The backing plates are typically secured, as noted, to the stationary component, for example, a diaphragm about a rotor in a steam turbine. The bristles extend from the backing plates typically at an angle offset from a radius of the rotor and in the direction of rotation of the rotor such that the free ends of the bristles engage the rotor and form a seal with the rotor. The bristles, in effect, comprise a plurality of cantilevered beams engaging along the surface of a rotating component. Brush seals sealing between a stationary component and a rotating component are also conventionally formed in brush seal segments. For example, six arcuate seal segments may be provided, each segment extending approximately 60xc2x0 and arranged end-to-end to form a complete 360xc2x0 seal about the rotating component.
As will be appreciated, severe flow conditions carry the risk of exciting one or more of the natural frequencies of the brush seal. In the event that a brush seal is excited during use at its resonant frequency, the bristles can break off adjacent their welded proximal ends, as well as along their free length, due to high-cycle fatigue. Because of this phenomenon, brush seal segments are typically tested in shaker tables to ascertain the natural frequencies of the seal, using stroboscopes to determine the natural frequencies of the brush seal segments as the frequencies are swept through. Because the bristles are packed one against the other in the seal, however, friction between the bristles themselves causes the natural frequency of the brush seal to change under various operating pressures, i.e., the differential pressure between regions on opposite sides of the seal. That is, as the pressure loading increases, bristles in engagement with one another and packed together, increase the seal stiffness, altering the natural frequencies of the seal.
Not only do the natural frequencies of the brush seal change under various pressure loadings but they also change as a function of rotor interference. That is, the force applied between the brush seal and the rotor also alters the natural frequencies of the seal. Accordingly, there is a need for apparatus and methods for testing brush seal segments to determine their natural frequencies under pressure loading and/or rotor interferences.
In accordance with a preferred embodiment of the invention, a pressure vessel which can be pressurized to the selected operating pressure level anticipated for the brush seal is mounted on a shaker table which sweeps through the frequency range of interest. A brush seal segment, preferably only a small arcuate segment, is mounted within the pressure vessel. An element having a radius simulating a rotor against which the tips of the bristles will engage when the brush seal is placed in use is mounted within the pressure vessel whereby the brush seal segment and element form a simulated seal. The seal is arranged in the pressure vessel such that regions on opposite sides of the seal are at different pressures. Viewports are provided in the pressure vessel for use, for example, with stroboscopes, to determine the natural frequencies of the brush bristles as the shaker table sweeps through the frequencies of interest.
With this arrangement, the natural frequencies of the brush seal at the anticipated operating pressure differential across the seal can be ascertained. Additionally, by adjusting the radial surface of the element toward or away from the bristle tips, the level of interference between the bristles and simulated rotor can be varied. As a consequence, the natural frequencies of the brush seal segment can be ascertained under varying pressures and/or interference fits between the bristles and the simulated rotor.
In a preferred embodiment of the invention, natural frequency testing apparatus for a brush seal comprises an enclosure having a mount, and an arcuate brush seal segment carried by the mount. The brush seal segment includes a plurality of bristles carried on a backing plate, with the bristles extending freely therefrom, terminating in bristle tips. An element within the enclosure has an arcuate surface for engaging the bristle tips and, together with the brush seal segment, defines a seal between two regions on opposite sides of the seal at different pressures, respectively. A connection carried by the enclosure couples the enclosure to a shaker which vibrates the enclosure and the brush seal segment mounted therein through a range of frequencies to ascertain a natural frequency of the brush seal segment under the differential pressure loading. Alternatively, the brush seal segment includes a plurality of bristles carried on a pair of backing plates, with the bristles extending freely therefrom, terminating in bristle tips.
In a further preferred embodiment of the invention, a method of determining a natural frequency of a brush seal segment having a plurality of bristles mounted on at least one backing plate with the bristles extending freely therefrom and terminating in bristle tips, comprises the steps of disposing the brush seal segment in a pressure vessel with the bristle tips engaging along an element so as to define a seal therewith situated between regions on opposite sides of the seal at different pressures, respectively, affording a pressure differential across the seal, and shaking the pressure vessel through a range of frequencies for determining a natural frequency of the brush seal segment at the differential pressure.
In a still further preferred embodiment of the invention, a method of determining a natural frequency of a brush seal segment having a plurality of bristles mounted on at least one backing plate, with the bristles extending freely therefrom and terminating in bristle tips, comprises the steps of disposing the brush seal segment in a fixture with the bristle tips engaging along an element so as to define a magnitude of interference therewith, and determining a natural frequency of the brush seal segment at the magnitude of interference between the brush seal segment and element.